1. Field of the Invention
The present invention relates to an image forming apparatus such as a photocopier and printer, particularly to an image forming apparatus for forming an image by correcting the position of transfer sheet in the direction perpendicular to the direction of conveying the transfer sheet.
2. Related Background Art
An image forming apparatus such as a photocopier and printer is provided with an image carrier, image writing section, development apparatus, sheet feed cassette, image transfer section, and fixing apparatus as well as an automatic conveyance apparatus for conveying sheets along a predetermined path. This automatic conveyance apparatus contains a plurality of conveyance rollers and motors, so that the transfer sheet is conveyed from upstream to downstream sides.
After an image has been formed on one side (front surface) of the transfer sheet, the transfer sheet is led through a reversing roller and reversing sheet conveyance path, whereby an image is formed on the other side (rear surface).
Further, to print a color image, four image forming units of yellow, magenta, cyan and black are arranged parallel to one another, and a toner image of each of the yellow, magenta, cyan and black colors formed sequentially by each of these image forming units is transferred to an image carrier on a primary basis. In this case, an intermediate transfer belt is often used as the image carrier. The toner image in the sense in which it is used here refers to the image formed on each photoreceptor of the corresponding image carrier by a developing device of each color. The toner image of each color is once transferred on an intermediate transfer belt (primary transfer), and a toner image is formed. After that, the image is transferred collectively from this intermediate transfer belt on transfer sheet (secondary transfer). Then the image formed on this transfer sheet is fixed, whereby a full-color or black-and-white image is formed.
In such an image forming apparatus, assume, for example, that misalignment occurs to the transfer sheet being conveyed to the image transfer section for transferring an image to the transfer sheet, and this misaligned transfer sheet is fed to the image transfer section. Then a misaligned image is transferred onto the transfer sheet. Thus, to prevent the image from being misaligned, transfer sheet must be fed to the image transfer section after correcting the misalignment of the transfer sheet.
In the image transfer section, arrival of the image (toner image) carried by the image carrier such as a photoreceptor drum and intermediate transfer belt must be timed with arrival of the transfer sheet conveyed by an automatic conveyance apparatus.
In one of the commonly known techniques to meet this requirement, a registration roller is provided on the upstream side of the image transfer section, and the leading edge of the transfer sheet is rotated by being brought in contact with the registration roller whose rotation has been stopped, whereby skewing of the transfer sheet is corrected. Further, to adjust the aforementioned timing in the conventional image forming apparatus, the aforementioned registration roller is used to control the time of arrival when the sheet is fed to the image transfer section. In one of the techniques using this control method (e.g. Patent Document 1), the position of the reference mark on the transfer sheet front surface and that on the image carrier are detected to estimate the amount of misalignment of the image marks on the front and rear surfaces in the sheet conveyance direction, thereby correcting the timing for feeding the transfer sheet on the registration roller. In another technique using this control method (e.g. Patent Document 2), immediately before a toner image on the rear surface is formed on the image carrier, the reference mark on the front surface of the transfer sheet is detected, whereby the timing for transfer is determined.
Further, in a conventional image forming apparatus, there is a further technique (e.g., Patent Documents 3 and 4) of adjusting the width-wise position of the transfer sheet perpendicular to the conveyance path wherein the reference mark of the transfer sheet front surface at some midpoint of a conveyance path is detected, and the timing of writing of the transfer sheet is corrected, whereby the position of the image in the main scanning direction and sub-scanning direction is corrected. In a still further technique (e.g., Patent Document 5), immediately before the toner image transferred to the rear surface of the transfer sheet is formed on the image carrier, the reference mark on the transfer sheet front surface is detected, thereby adjusting the position of the image formed on the image carrier in response to the position of the reference mark of the transfer sheet front surface at that time.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 2004-279749
[Patent Document 2] Unexamined Japanese Patent Application Publication No. 10-319674
[Patent Document 3] Unexamined Japanese Patent Application Publication No. 2000-305324
[Patent Document 4] Unexamined Japanese Patent Application Publication No. 2003-156974
[Patent Document 5] Unexamined Japanese Patent Application Publication No. 10-115954
However, while the transfer sheet is conveyed, transfer sheet misalignment may occur across the width of the transfer sheet perpendicular to the direction of conveyance (hereinafter referred to as “across the width”). Further, when an intermediate transfer belt is used as an image carrier, misalignment occurs to the image on the aforementioned intermediate transfer belt across the width due to coasting of the aforementioned intermediate transfer belt. Thus, when using a conventional device of adjusting the position across the width by reference mark position immediately before image formation on the conventional image carrier, it is necessary to increase the distance from detection of the reference mark to the transfer by the number of colors in the color printing mode. Accordingly, sheet and image carrier will be misaligned across the width due to conveyance and coasting during this time. This makes it difficult to make high-precision adjustment of the image position across the width. In the method of correction by adjusting the position in the direction of conveying a toner image formed on the image carrier and across the width without moving the transfer sheet per se, much time is required to correct the misalignment in the direction of conveyance of the image on the carrier for each page and misalignment across the width and to adjust the positions of the images on pages printed on a continuous basis.